There are two kinds of isomers, cis- and trans-isomers of 1,2-diphenylethylene analogues. Because the cis-isomer has higher energy than that of the trans-isomer, 1,2-diphenylethylene analogues normally exists in the form of trans-isomer with a more stable configuration. In general, the synthetic routes of 1,2-diphenylethylene analogues are Wittig reaction or modified Wittig reactions, condensation between carbon anion and hydrazone obtained from aromatic aldehydes, reductive coupling of carbonyl compound or self-coupling of alpha-lithiumbenzylphenylsulfone to prepare trans-isomer of 1,2-diphenylethylene analogues predominantly rather than the cis-isomer. However, the cis-1,2-diphenylethylene analogues is different from the trans ones in both physical and chemical properties. As the U.S. Pat. No. 5,525,632 disclosed, the combrestatin with the structure of cis-1,2-diphenylethylene analogues has carcinostatics activity. And the Chinese patent CN1732884A reported that cis-1,2-diphenylethylene analogues can be used to prevent and to treat diabetes. Therefore, it is important to develop a method for the production of cis-1,2-diphenylethylene analogues.
The isomerization occurs between the two isomers under direct irradiation of lights. Hammond, G., and Satiel, J. reported that, when the singlet 1,2-diphenylethylene reaches reaction equilibrium under irradiation with ultraviolet light, the ratio of cis- to trans-isomers on the photostationary state is dependent on the different molar extinction coefficient of the isomers under the light wavelength used (J. Am. Chem. Soc. 1964, 86, 3197; J. Am. Chem. Soc. 1967, 89, 1036; J. Am. Chem. Soc. 1972, 94, 2742). Nevertheless, the quantum yield of the reaction is very low. Then they used the small molecules like benzophenone as photosensitizers to sensitize the trans-1,2-diphenylethylene to its triplet state, followed by isomerization to its cis-isomer. On the photostationary state, the ratio of cis- to trans-isomers is dependent on the triplet energy of the sensitizer used (J. Am. Chem. Soc. 1963, 85, 2515; J. Am. Chem. Soc. 1968, 90, 1899; J. Am. Chem. Soc. 1968, 90, 6394; J. Am. Chem. Soc. 1972, 94, 3449; J. Am. Chem. Soc. 1984, 106, 3144; J. Am. Chem. Soc. 1971, 93, 532). However, the short lifetime of the sensitizer and weak interaction between the sensitizer in excited state and the 1,2-diphenylethylene, together with the sensitizer bleaching during the reaction lead to low quantum yield of sensitization, long reaction time and complicated reaction products. To facilitate the photoisomerization, Whitten, D. G., et al introduced metalloporphyrin complexes such as zinc porphyrin and bipyridyl ruthenium complexes as sensitizers to reach the trans-isomer predominantly with high rate acceleration. (J. Am. Chem. Soc. 1972, 94, 7811; J. Am. Chem. Soc. 1973, 95, 4367; J. Am. Chem. Soc. 1978, 100, 2620). Recently Wilcox et al. covalently linked [Ru(bpy)3]2+ or benzil and 1,2-bis(biphenyl)ethene units to investigate the photoisomerization kinetics and found that the linked sensitizer increases the rate of isomerization and significantly changes the photostationary state (J. Am. Chem. Soc. 2006, 128, 250; J. Am. Chem. Soc. 2007, 129, 3966). Under specific condition, 1,2-diphenylethylene analogues can undergo one-way isomerization upon irradiation with light. Eenkhoorn et al. found that the one-way isomerization from trans-isomer to cis-isomer of singlet 1-(2-pyrrolyl)-2-(2-indolyl)ethene occurred (Can. J. Chem. 1973, 51, 792.). Lewis et al. also illustrated that the one-way isomerization is due to the existence of intramolecular hydrogen bond in ground state of the cis-isomer (J. Am. Chem. Soc. 1995, 117, 3029). In 1983 Arai, Tokumaru et al. reported that the triplet olefin substituted by styrylanthracene underwent one-way isomerization from cis-isomer to trans-isomer (Tetrahedron Lett. 1983, 24, 2873).
Taken all together, it is evident that there has been a great effort to study the isomerization of 1,2-diphenylethylene analogues since the first report of isomerization of olefin in 1931 (Trans. Faraday Soc., 1931, 27, 69). Despite of this, examples related to the preparation of cis-1,2-diphenylethylene analogues are quite few. In the present invention polypyridyl platinum (II) complexes are adopted as a highly effective photocatalyst, to accelerate the trans-cis photoisomerization of 1,2-diphenylethylene analogues by using visible light, to achieve the cis-1,2-diphenylethylene analogues or cis-1,2-diphenylethylene analogues predominantly at photostationary state.